Temperature increase dynamics reactors

In cases where a large reactor operates similarly to a CSTR, fluid dynamics sometimes can be estabflshed in a smaller reactor by external recycle of product. For example, the extent of soflds back-mixing and Hquid recirculation increases with reactor diameter in a gas—Hquid—soflds reactor. Consequently, if gas and Hquid velocities are maintained constant when scaling and the same space velocities are used, then the smaller pilot unit should be of the same overall height. The net result is that the large-diameter reactor is well mixed and no temperature gradients occur even with a highly exothermic reaction. 

Besides these purely static aspects, the dynamic behavior of an adiabatic batch reactor must also be considered. The adiabatic temperature course is a function of the thermal properties of the reaction mixture. The adiabatic temperature increase influences the final temperature as well as the rate of the temperature increase. For highly exothermal reactions, even for small increase in conversion, the increase in temperature is important (see Section 2.4.3). 

There are several other aspects about CSTRs with exothermic reactions that should be mentioned at this point. The first involves the temperature of the feed. The colder the feed, the less heat must be transferred from the reactor. So control would be expected to be improved. However, as we will see in Chapter 3, a cold feed can produce some interesting dynamics for instance, an increase in feed flowrate initially decreases reactor temperature because of the sensible-heat effect. But as the reactant concentration in the reactor increases, the temperature eventually increases. A reactor temperature runaway can result if the cold feed quenches the reaction and reactant concentration builds to a very high level before the reaction lights off. ... 

The feed temperature is changed from 400 to 420 K by selecting the feedstream and clicking Forms and Manipulate, as shown in Figure 6.38. Dynamic response results are shown in Figure 6.39. The feed temperature is increased 20 K at time = 0.1 h. The exit temperature of the reactor without catalyst (Tout)nocat increases from 582 to 595 K very quickly since the gas residence time in the reactor is only 20 s. 

The dynamic responses of the internal temperature help to explain this unusual phenomenon. When the inlet temperature increases, it takes some time for the temperature at 4 m (20% down the length of the 20-m reactor) to begin to increase. When this occurs, more of the chlorine is consumed in this section of the reactor. This reduces the chlorine concentration in the gas flowing through the rest of the catalyst bed, which reduces the... 

The process is subjected to a number of disturbances, and the control structure handles all of them quite effectively. Dynamic responses to changes in the setpoint of the temperature controller in the first reactor are shown in Figure 6.109. At 0.1 h, the setpoint is increased from 245 to 255°C. At 3 h, it is decreased to 235°C. Decreasing the temperature in the first reactor results in an increase in throughput. The synthesis gas feedrate, the product rate, and the vent rate all increase. The opposite occurs when the temperature is increased. This indicates that the reaction is equilibrium-limited, not kinetically limited. Decreasing temperature increases the equilibrium constant of exothermic reactions. 

The dynamic simulations of the reactor by itself are shown in Figure 7.12. A very small increase in reactor inlet temperature produces a reactor runaway because of the high activation energy and high heat of reaction. Compare these results with those of the base case kinetic parameters shown in Figure 1.9b. 

These results indicate that a process change would probably be required to handle the dynamic problems. There are several alternatives. A cooled nonadiabatic reactor should reduce the sensitivity since more heat will be removed as temperatures increase. Probably a more practical solution would be to design for a lower concentration of one of the reactants. This mode of operation would prevent reaction runaways because the reaction rate would drop olf quickly as the concentration of the limiting reactant declined. The economic penalties would include requiring a larger reactor and more recycle than in the equimolar pure reactant feed mode of operation. Alternatively, the concentrations of both reactants could be reduced by recycling an inert substance (probably product C). This would also increase reactor size and recycle flowrate. 

The reactor model available in Aspen Dynamics [16] only provides the possibility of changing the coolant temperature. Figure 10.12 shows results dynamic simulation results, for the following scenario the plant is operated at the nominal steady state for lh. Then, the coolant temperature is increased from 413 to 425 K and simulation is continued for 2 h. The maximum temperature inside the reactor... 

Figure 9.4 Irreversible reaction case dynamic response to 10 F increase in reactor inlet temperature.

Figure 11.5 Dynamic response of base case to 6 C increase in reactor temperature.

Increasing the reactor temperature setpoint increases the production rate of vinyl acetate, so there must ultimately be net increases in all three fresh reactant feed streams. Oxygen and ethylene flows respond fairly quickly within about 20 minutes. However, the acetic acid feed actually decreases for the first 60 minutes in response to an increase in column base level. These results demonstrate the slow dynamics of the liquid recycle loop and illustrate the need for controlling the total acetic acid flow to the reactor so that the separation section does not see these large swings in load ( snowball effect"). The variability is absorbed by the fresh feed makeup stream. 

Introduction of CO + (760 Torr) in a molar ratio 2 1 in the glass equipment was followed by a stepwise increase of temperature from 25 up to 200°C. Analysis of the gas phase gave the results represented on Figure la. At 176°C the conversion of CO to hydrocarbons is close to 1 % with mainly propylene (32%), methane (26,1 %) ethylene (9,2 %), 1-butene (7,3 %), cis-2-butene (3,6 %), trans-2-butene (5,5 %), isobutene (1 %) and C, hydrocarbons (7 %). All the paraffins except methane are present in much smaller amount than olefins. Figure (lb) represents typical results obtained in Fischer-Tropsch synthesis in a dynamic reactor using a catalyst derived from Fe CO) jj/A Oj (3e). 

The first technical problem of a reactor is its criticality. It was possible to think, at one time, that the calculation of the nuclear behavior of a reactor had been reduced to standard operations. Bight now, we do not know whether this is true. It is certain, however, that the standard operations of the present time are very different from the standard operations of five or ten years ago. The last five years or so brought an extension of the reactor theory to nonhnear cases, that is, to power densities high enough to affect the reactor s properties in periods comparable with the natural period of the reactor, that is comparable with the lifetimes of the delayed neutron emitters. If this situation prevails— and it was present in some reactors which were in operation more than ten years ago—the dynamic behavior of the reactor does not obey a linear equation any more. There are effects which are proportional to the square of the neutron density because they depend not only on the number of neutrons but also on the temperature increase created by these neutrons. A characteristic of such a behavior is the deviation from the exponential or sine-like time dependence of the neutron densities. 

Differently from the interconnected fluidized bed reactors, the dynamically operated PBRs have to be operated with proper heat management strategies in order to produce hot gas that are suitable for a combined cycle. In a PBR, the maximum solid temperature, and, thus the maximum gas temperature for the thermodynamic cycle, is usually achieved during the oxidation phase (which is always strongly exothermic as shown in Figure 5.15) and the maximum solid temperature increase is caleulated as follows [61] ... 

Accidents happening in polymerization reactors are practically always due to a lack of control of the course of reaction caused by a disturbance of the heat balance, which results in a temperature increase leading to loss of control of the reactor and a runaway reaction. In this section a systematic procedure based on a failure scenario with six key questions, allowing assessment of the criticality of a process, is presented. Since the heat balance is at the center of our concerns in matters of thermal control of reactors, the different terms of the heat balance will be examined. Finally, aspects of the dynamic stability of reactors and of the thermal stability of reaction masses are analyzed. 

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